1. Field of the Invention
The present invention generally relates to systems for control of a gas appliance and more particularly relates to electronic control of the main burner.
2. Description of the Prior Art
It is known in the art to employ various appliances for household and industrial applications which utilize a fuel such as natural gas (i.e., methane), propane, or similar gaseous hydrocarbons. Typically, such appliances have the primary heat supplied by a main burner with a substantial pressurized gas input regulated via a main valve. Ordinarily, the main burner consumes so much fuel and generates so much heat that the main burner is ignited only as necessary. At other times (e.g., the appliance is not used, etc.), the main valve is closed extinguishing the main burner flame.
A customary approach to reigniting the main burner whenever needed is through the use of a pilot light. The pilot light is a second, much smaller burner, having a small pressurized gas input regulated via a pilot valve. In most installations, the pilot light is intended to burn perpetually. Thus, turning the main valve on provides fuel to the main burner which is quickly ignited by the pilot light flame. Turning the main valve off, extinguishes the main burner, which can readily be reignited by the presence of the pilot light.
These fuels, being toxic and highly flammable, are particularly dangerous in a gaseous state if released into the ambient. Therefore, it is customary to provide certain safety features for ensuring that the pilot valve and main valve are never open when a flame is not present preventing release of the fuel into the atmosphere. A standard approach uses a thermogenerative electrical device (e.g., thermocouple, thermopile, etc.) in close proximity to the properly operating flame. Whenever the corresponding flame is present, the thermocouple generates a current. A solenoid operated portion of the pilot valve and the main valve require the presence of a current from the thermocouple to maintain the corresponding valve in the open position. Therefore, if no flame is present and the thermocouple(s) is cold and not generating current, neither the pilot valve nor the main valve will release any fuel.
In practice, the pilot light is ignited infrequently such as at installation, loss of fuel supply, etc. Ignition is accomplished by manually overriding the safety feature and holding the pilot valve open while the pilot light is lit using a match or piezo igniter. The manual override is held until the heat from the pilot flame is sufficient to cause the thermocouple to generate enough current to hold the safety solenoid. The pilot valve remains open as long as the thermocouple continues to generate sufficient current to actuate the pilot valve solenoid.
The safety thermocouple(s) can be replaced with a thermopile(s) for generation of additional electrical power. This additional power may be desired for operating various control circuitry of equipment auxiliary to the gas appliance. Normally, this requires conversion of the electrical energy produced by the thermopile to a voltage useful to these additional loads. Though not suitable for this application, U.S. Pat. No. 5,822,200 issued to Stasz; U.S. Pat. No. 5,804,950, issued to Hwang et al.; U.S. Pat. No. 5,381,298, issued to Shaw et al.; U.S. Pat. No. 4,014,165, issued to Barton; and U.S. Pat. No. 3,992,585, issued to Turner et al. all discuss some form of voltage conversion.
Upon loss of flame (e.g., from loss of fuel pressure), the thermocouple(s) ceases generating electrical power and the pilot valve and main valve are closed, of course, in keeping with normal safety requirements. Yet this function involves only a binary result (i.e., valve completely on or valve completely off). Though it is common within vehicles, such as automobiles, to provide variable fuel valve control as discussed in U.S. Pat. No. 5,546,908, issued to Stokes, and U.S. Pat. No. 5,311,849, issued to Lambert et al., it is normal to provide static gas appliances with a simple on or off valve.
Yet, there are occasions when it is desirable to adjust the main burner supply valve of a standard gas appliance. These include changes in mode (i.e., changes in the desired intensity of the flame) and changes in the fuel type (e.g., change from propane to methane). Whereas some appliances have manual valves, it would be desirable to have electronically controlled valves.
The present invention overcomes the disadvantages of the prior art by providing a main burner valve for a gas appliance which is precisely controllable. Furthermore, the present invention has a valve system totally powered from the pilot light flame. The valve assembly of the present invention is electronically monitored to ensure proper operation and to conserve electrical power.
In accordance with the preferred mode of the present invention, a thermopile is thermally coupled to the pilot flame. As current is generated by the thermopile, it is converted via a DC-to-DC converter to a regulated output and an unregulated output. The regulated output powers a microprocessor and other electronic circuitry which control operation of the main fuel valve, remote communication with the operator, and speed of the circulating fan. The unregulated output powers various mechanical components including a stepper motor which controls the main burner valve.
The stepper motor is mechanically coupled to a linear actuator which precisely positions the main fuel valve. The use of the stepper motor means that any selected valve position is held statically by the internal rachet action of the stepper motor without quiescent consumption of any electrical energy. That makes the electrical duty cycle of the stepper motor/valve positioning system extremely low. This is a very important feature which permits the system to operate under the power of the thermopile without any necessary external electrical power source. In fact, the stepper motor duty cycle is sufficiently low, that the power supply can charge a capacitor slowly over time such that when needed, that capacitor can power the stepper motor to change the position of the linear valve actuator.
A particularly important feature of the present invention is the monitoring of the stepper motor operation. When the system tries to move the stepper motor, it does not have any feedback to confirm stepper movement. There are three reasons why detection of stepper movement is needed:
1. When performing the self-calibration (see electronic convertibility disclosure), it is necessary to drive the stepper to a hard stop position. As there is no motion or position detector (to save cost and power), the present invention is capable of detecting at which driving pulse stepper stops movement.
2. During normal flame height modulation, as the stepper is driven with a voltage lower than specified voltage, the stepper may stop. It is necessary to detect any slippage.
3. As soon as stepper movement is detected, the driving pulse may be terminated early to conserve power.
The stepper motor has four stator coils arranged about the four rotational quadrants. For most movements of the stepper motor, only a single coil is utilized and in no case are more than two used. Thus, whenever the stepper motor is in motion, at least two of the four coils are unused. An electrical potential is induced into the unused coils as a result of the rotational movement of the permanent rotor magnets of the stepper motor. By monitoring these signals and analyzing them, certain characteristic signatures are revealed. Thus, the microprocessor can verify that motion has actually occurred, can determine the direction of the motion, and can ascertain when the motion has been completed. In this way, the microprocessor can increase safety and decrease power consumption.